Motor nerves play the critical role of shunting information out of the central nervous system to targets in the periphery. Their formation requires the coordinated development of distinct cellular components, including motor axons and the glial cells that ensheathe them. During nervous system construction, these cells must migrate long distances and coordinate their differentiation, ensuring the efficient propagation of electrical information. To better understand, diagnose and treat the many degenerative disorders of the peripheral nervous system, we need to comprehend the cellular and molecular mechanisms that mediate glial interactions along developing nerves and myelin maintenance in juvenile and adult organisms. Zebrafish provide a unique opportunity to directly observe and manipulate cell populations to gain insight into how the PNS is initially established, maintained and behaves during disease. In preliminary studies, we demonstrate that perineurial cells, which form the perineurium, originate as glial cells in the ventral spinal cord, influence Schwann cell development and are essential to motor nerve development. Therefore, in Aim 1 of this project, we will investigate the hypothesis that Schwann cells and perineurial glia reciprocally interact during nerve assembly by utilizing time-lapse imaging and genetic manipulation. In Aim 2, we will characterize two new mutant lines that have defects in perineurial glial development and identify the mutated genes responsible for the phenotypes. Identifying these genes will give us additional information about the molecular mechanisms necessary for PNS formation. Completion of these aims will greatly expand our knowledge of the cellular and molecular mechanisms that mediate peripheral nerve development, facilitating new drug therapies intended to treat peripheral neuropathies.